It is known that image projectors may comprise a spatial light modulator, a light source, which is arranged to emit white light, and a rotating filter wheel. The spatial modulator is sequentially illuminated with light having different colors so as to display a multi-color image. The sequentially changing color of the illuminating light is provided by filtering white light by the rotating filter wheel. However, the filtering wastes optical power.
It is known to use three separate light sources and three separate spatial light modulators may be used, wherein each light source is arranged to illuminate its own light modulator with a different color. The colors may be e.g. red, green, and blue (RGB). Images formed by the different spatial light modulators may be combined so as to form a single multi-color image.
The spatial light modulator may be e.g. a LCD array (Liquid crystal), an LCoS array (Liquid Crystal on Silicon), or a MEMS array (Micro Electro-mechanical System) comprising e.g. a plurality of micromechanically movable miniature mirrors.
Light beams of three separate light sources may be combined into a single combined beam by using a beam combiner, which comprises wavelength-selective mirrors. The combined beam may be arranged to illuminate a spatial light modulator. The color of the combined beam may be sequentially changed by modulating the optical power of the separate light sources. The wavelength selective mirrors of the beam combiner may be implemented by multilayer coatings.
However, several different materials and manufacturing processes may be needed to produce the wavelength-selective mirrors. Furthermore, tolerances for positioning the wavelength-selective mirrors with respect to the light sources may be tight.